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Did Neurons Evolve from Secretory Cells?

  1. Jan 11, 2019 at 9:41 PM #1

    BillTre

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    People in this Science magazine news article argue yes.
    The idea that the neuron cell type was in some way derived from secretory cells has been around for a while (1970's at least).

    The new findings involve finding similarities in how nerve cells and particular secretory cells shared an embryonic cell precursor, thus in a way a common developmental source.
    Similarly, they argue, particular neurons and secretory cells share developmental precursors in coelenterates (jellyfish and hydra) and fruitflies.
    This argument is used to unite the neurons of ctenophores (comb jellies) with the neurons found in all the other animals (except porifera (sponges) and placozoans (small blobs with maybe 4 cell types).

    Based on the phylogeny of early animals one uses, some will claim the ctenophore neurons require a independent evolution on neurons, which would involve a lot of parallel evolution of molecular components and their integration (see below).
    The independent origin a ctenophores neurons was originally supported by finding that the ctenophore genome lacked some transmitter synthesis enzymes and some nervous system expressed Hox genes.

    Alternatively, losses of the neuron cell type in the placozoans and porifera evolutionary lineages may, perhaps more easily, explain the phylogenic distribution of neurons among the phyla, assuming the origin of neurons in the metazoan final common ancestor. this is something discussed in the article.

    This old idea of an evolutionary relationship between neurons and secretory cells is based upon the many details of the complex secretory system of secretory cells that are shared with neurons and that reside in the molecular secretory system. Molecules of the secretory system does the following:
    • producing vesicles (small lipid enclose balls with an chewy aqueous center) with in the cytoplasm of the cell
    • filling the vesicles with the appropriate chemicals/biologicals
    • cellular mechanisms that keep the vesicle in their proper cellular location (for release)
    • cellular mechanisms the cause the vesicle membranes to fuse with the cellular membrane so that the vesicle's contents are released from the cell (or secreted)
    • A triggering mechanism so that the release is appropriate to the cell's conditions
    These mechanisms will involve the activity of many different genes working together.
    Not a trivial evolutionary accomplishment.

    Neurons took these functions of secretory cells and added:
    • cytologically distinct cell sub-regions (axons, dendrites, maybe derived from something like the regional difference between apical and basal ends of an epithelial cell (a cell type that shows up earlier in evolution)).
    • transport mechanisms to supply the distal ends of the cell's processes
    • electrochemical mechanisms (mostly ion channel molecules and pump molecules), sorted out to the appropriate parts of the cell
    • in-coming and out-going synapse mechanisms
      out-going: using the secretory mechanisms to release vesicle contents
      in-coming: using concentrating receptors and transmitter molecule removal mechanisms
      cellular mechanisms will have to ensure the in-coming and out-going synaptic components of adjacent cells are matched up in space.
    These ideas about these relationships of among different cell types might be further studied by comparing their transciptomes (sets of genes each cell is expressing).
     
  2. jcsd
  3. Jan 12, 2019 at 10:44 AM #2

    Buzz Bloom

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    Hi Bill:

    I recall from something I read a long time ago that Lynn Margulis suggested that the axons and dendrites in nerve cells evolved from the spindle fibers used in cell reproduction. This also explains why nerve cells do not reproduce. I think she discussed tubulin as the relevant protein, and which is also a component of cilia.
    I apologize for my not being able to cite the book I originally read.

    Regards,
    Buzz
     
  4. Jan 12, 2019 at 5:11 PM #3

    BillTre

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    She is right Buzz, that microtubules are used in both normal cells as well as axons and dendrites.
    They are involved in structural, movement and internal cellular transportation of cell components in many cells.
     
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